When performing still or video imaging of moving subjects, for example, in low lighting (for example, when taking indoor pictures of children), the images are often distorted, due to blurring caused by movement and a long exposure time. An example of a shot according to the prior art is shown in FIG. 8d, in which the stationary subject has been imaged by moving the camera vertically.
In the prior art, the total exposure of a shot is determined using the exposure time and analog and digital gain. The aperture size too can be adjusted, if it is an adjustable type. However, particularly mobile devices lack the possibility to adjust the aperture. Lower lighting demands a longer exposure time. Unfortunately, a longer exposure time will increase blurring caused by a moving subject, or camera shake at the moment of shooting/in imaging. Analog gain can be used to compensate for part of the exposure time required. In that, for example, doubled analog gain will half the necessary exposure time, but at the same time, however, the noise level will increase.
Analog gain amplifies the noise that has already been generated in the image. Nevertheless, it can be used to reduce quantization noise that occurs in connection with image capture, which in the worst case appears as the total loss of levels from the image. Digital gain, which is used along with analog gain, does not in reality bring additional information to the image. Digital gain only simply amplifies the image signal and the noise, which are results of the exposure time used and of analog gain. In addition, the possibilities to use analog gains and exposure times and their feasibilities vary from one sensor to another.
In the AEC (Automatic Exposure Control) method according to the prior art, the AEC mode of a digital camera typically sets the exposure time in such a way that, at the maximum, only just amount analog gain is used to prevent the noise level of the analog gain from reaching a disturbing level. In low-lighting conditions, this causes serious blurring, particularly in moving subjects (the actual level of lighting, at which the problem begins to appear, depends on the properties of the camera sensor). A large amount of motion-blurring will destroy the details in the image even more than the noise arising from use of analog gain.
The prior art knows different kinds of user-selectable exposure programs from several camera devices, which can be called, for example, ‘sport programs’ of exposure, if the program is intended for difficult exposure conditions of the type described above (for example, for imaging moving subjects). In these, greater analog/digital gain can be permitted to prevent motion blurring, so that the exposure times are, in turn, kept shorter. A sport program cannot, however, be used as a default program, as it may cause an unnecessarily high level of noise, for example, in bright lighting conditions, or in low lighting conditions, in which there are no moving subjects.
Entirely manually adjusted exposure programs are also known. In these, the end user can select a precise exposure time and analog gain (which is typically called an ISO setting, a name inherited from traditional cameras) and thus make a compromise between motion blurring and noise.
Flash lighting can also be used as a means to solve the problem in the case of still images, but it cannot be applied to video. In addition, some digital cameras and particularly mobile terminals equipped with digital cameras, completely lack flash lighting, due to considerations of space/price/power consumption.